1. Field of the Invention
The present invention relates to a device for controlling a charging pressure of an internal combustion engine.
2. Description of the Related Art
In an internal combustion engine equipped with a mechanically driven charger used for increasing the pressure of air fed into the cylinders, that is, increasing the charging pressure, the charging pressure is increased as the engine speed is increased, and thus the amount of air fed into the engine cylinders becomes maximum when the engine is operating under a heavy load at a high speed. Consequently, when the engine is operating under a heavy load at a high speed, the thermal load of the engine becomes maximum, and thus, if the charging pressure is excessively increased at this time, the durability of the engine is decreased. Therefore, in an engine equipped with a mechanically driven charger, it is necessary to control the charging pressure to a pressure lower than a charging pressure which will have an adverse affect on the engine when the engine is operating under a heavy load at a high speed, wherein the engine load becomes maximum. To this end, an engine is known in which a throttle valve is arranged in the intake passage of the engine, and a mechanically driven charger driven by the engine is arranged in the intake passage downstream of the throttle valve. A bypass passage is branched from the intake passage between the throttle valve and the mechanically driven charger and connected to the intake passage downstream of the mechanically driven charger. A relief valve having a pressure control chamber therein is arranged in the bypass passage, and the pressure control chamber of the relief valve is connected to the intake passage between the throttle valve and the mechanically driven charger, so that the relief pressure of the relief valve is decreased as the level of vacuum acting on the pressure control chamber is increased (see Japanese Unexamined Utility Model Publication No. 61-17138). In this engine, when the discharge pressure of the mechanically driven charger, i.e., the charging pressure, exceeds the relief pressure of the relief valve, a part of air pressurized by the mechanically driven charger is returned to the intake passage upstream of the mechanically driven charger via the bypass passage, and as a result, the charging pressure is maintained at a predetermined relief pressure. As mentioned above, in this engine, since the charging pressure is controlled to a pressure that does not exceed the predetermined relief pressure, it is possible to prevent a decrease of durability of the engine.
Another engine is known in which a throttle valve is arranged in the intake passage of the engine, and a mechanically driven charger driven by the engine is arranged in the intake passage downstream of the throttle valve. A bypass passage is branched from the intake passage between the throttle valve and the mechanically driven charger and connected to the intake passage downstream of the mechanically driven charger, and a control valve having a pressure control chamber therein is arranged in the bypass passage. When the temperature of the engine is low, the pressure control chamber of the control valve is open to the outside air so that the control valve is kept shut. After the warm-up of the engine is completed, the pressure control chamber of the control valve is connected to the intake passage between the throttle valve and the mechanically driven charger. At this time, when the level of vacuum in the intake passage between the throttle valve and the mechanically driven charger is reduced, that is, when the engine is operating under a heavy load, the control valve is closed (see Japanese Unexamined Utility Model Publication No. 61-17141). In this engine, when the temperature of the engine is low, the charging pressure is maintained at a maximum charging pressure which can be obtained by the mechanically driven charger by keeping the control valve shut irrespective of the engine load, thus obtaining an easy engine start up.
As mentioned above, the thermal load of the engine becomes maximum when the engine is operating under a heavy load at a high speed, and thus the durability of the engine depends on the change in pressure produced when the engine is operating under a heavy load at a high speed. Conversely, when the engine is operating at a middle or low speed, the thermal load of the engine is low. Consequently, at this time, even if the charging pressure is increased, there is no danger of a decrease in the durability of the engine. On the contrary, by increasing the charging pressure, it is possible to obtain a good accelerating operation.
However, if the pressure control chamber of the relief valve is connected to the intake passage between the throttle valve and the mechanically driven charger, and if the relief pressure is controlled to a low pressure when the engine is operating under a heavy load at a high speed, as in an engine disclosed in the above-mentioned Japanese Utility Model Publication No. 61-17138, the relief pressure is inevitably controlled to a low pressure when the engine is operating at a middle speed. As a result, since the charging pressure is controlled to a low pressure when the engine is operating at a middle speed, it is difficult to obtain a good accelerating operation.
In an engine disclosed in the above-mentioned Japanese Utility Model Publication No. 61-17141, the charging pressure is not controlled to a low pressure when the engine is operating under a heavy load at a high speed. Consequently, in this engine, the problem to be solved by the present invention does not essentially arise.